The effect of Oenothera biennis (Evening primrose) oil on inflammatory diseases: a systematic review of clinical trials

Background Evening primrose oil (EPO), extracted from the seeds of Oenothera biennis, has gained attention for its therapeutic effects in various inflammatory conditions. Method We performed a systematic search in multiple databases and defined the inclusion criteria based on the following PICOs: P: Patients with a form of inflammatory condition, I: EPO, C: Placebo or other therapeutic interventions, O: changes in inflammatory markers or patients’ symptoms; S: randomized controlled trials. The quality of the RCTs was evaluated using Cochrane’s RoB tool. Results Several conditions were investigated in the literature. In rheumatoid arthritis, mixed results were observed, with some studies reporting significant improvements in symptoms while others found no significant impact. EPO showed some results in diabetes mellitus, atopic eczema, menopausal hot flashes, and mastalgia. However, it did not demonstrate effectiveness in chronic hand dermatitis, tardive dyskinesia, psoriatic arthritis, cystic fibrosis, hepatitis B, premenstrual syndrome, contact lens-associated dry eyes, acne vulgaris, breast cyst, pre-eclampsia, psoriasis, or primary Sjogren's syndrome. Some results were reported from multiple sclerosis after EPO consumption. Studies in healthy volunteers indicated no significant effect of EPO on epidermal atrophy, nevertheless, positive effects on the skin regarding hydration and barrier function were achieved. Conclusion Some evidence regarding the potential benefits of EPO in inflammatory disorders were reported however caution is due to the limitations of the current survey. Overall, contemporary literature is highly heterogeneous and fails to provide strong recommendations regarding the efficacy of EPO on inflammatory disorders. Further high-quality studies are necessitated to draw more definite conclusions and establish O. biennis oil effectiveness as an assuring treatment option in alleviating inflammatory conditions. Supplementary Information The online version contains supplementary material available at 10.1186/s12906-024-04378-5.


Introduction
Inflammatory diseases, characterized by chronic inflammation and immune dysregulation, continue to pose significant challenges in healthcare, with their prevalence and impact on global health steadily increasing.Inflammation is a complex biological response that plays a crucial role in the pathogenesis of numerous diseases, including rheumatoid arthritis (RA), diabetes mellitus (DM), atopic eczema (AE), and many others [1][2][3].Conventional therapies for inflammatory diseases often involve the use of nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, immunosuppressants, or biologic agents.However, these treatments may be associated with adverse effects, limited efficacy, or high costs, necessitating exploring of alternative therapeutic options.Traditional medicine, rooted in centuries-old practices and knowledge systems and including the use of herbal remedies, has emerged as a valuable resource in the management of inflammatory diseases [4,5].As far as we know, synthetic and semi-synthetic pharmaceutical derivatives from plants are being used in most clinical drugs and with the spread of various diseases, hundreds of plant-based molecules are ongoing to be discerned and discovered [6].Given that, natural compounds have also been used in the anti-inflammatory diseases [7,8].Evening primrose oil (EPO), derived from the seeds of Oenothera biennis, has gained attention for its potential therapeutic effects in various inflammatory diseases [9].EPO is rich in essential fatty acids(EFAs), including linoleic acid (LA) and gamma-linolenic acid (GLA), which is a precursor for anti-inflammatory substances in the body (6) [9].GLA is metabolized into prostaglandin E1 (PGE1), a potent anti-inflammatory mediator that can modulate immune responses and reduce inflammation.Additionally, EPO contains other bioactive compounds, such as flavonoids and phenolics, which possess antioxidant and anti-inflammatory properties.The identified polyphenols and flavonoids is previous studies were mainly gallic acid, caffeic acid, epicatechin, coumaric acid, ferulic acid, rutin and rosmarinic acid [9].In another study again, ( +)-catechin, (-)-epicatechin and gallic acid were reported to be the main components of EPO phenolic compounds [10] and one isoflavone together with 2-hydroxychalcone were revealed in EPO [11].
Lipoxygenase (LOX) and cyclooxygenase (COX) are the two pro-inflammatory enzymes that synthesize the eicosanoids (like leukotriene (LT), prostaglandin (PG)) from arachidonic acid (AA), and therefore, playing an essential role in inflammatory processes.Based on aforementioned statement, EPO components, showed antiinflammatory activities via inhibition of LOX [3,12].It is reported that EPO was beneficial in different types of inflammatory diseases.To assess the potential benefits of EPO in inflammatory diseases, a systematic review of relevant clinical trials was conducted.The primary objective was to synthesize the available evidence and critically evaluate the effectiveness of EPO in improving symptoms, reducing disease activity, and enhancing overall outcomes in inflammatory conditions.

Methodology
The current literature protocol is registered in the International Register of Prospective Systematic Reviews (PROSPERO ID: CRD42023394200).This systematic review and meta-analysis was conducted using the accepted systematic review method of the book entitled "A Systematic Review for Evidence-Based Support Medicine" [13] and according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [14].

Information sources and search strategy
As shown in Table 1, proper search terms were defined based on the PICOs and combined using Boolean operators to produce each search string.The following databases were searched: Google Scholar, PubMed, EMBASE, Scopus, and Cochrane Central Register of Controlled Trials.The search was carried out by two independent reviewers in the end of December 2022.Some of the applicable journals and websites were also searched manually.Reference lists of the chosen articles were reviewed as well.

The inclusion and exclusion criteria
The inclusion and exclusion criteria were defined in accordance to the PICOs, which is defined in the Table 1.
Inclusion criteria are as follows: a) Studies with adult participants (≥ 18 years) suffering from an inflammatory disease of any nature, including rheumatologic, gastrointestinal, cardiovascular, metabolic, etc. b) Studies using Oenothera biennis (Evening primrose) oil as an intervention.c) Randomized controlled trials (RCTs).d) Published in English.
Exclusion criteria were: a) Using any other form of intervention in the absence of evening primrose oil b) Animal studies and basic experiments.c) Unoriginal publications, reviews, overviews, letters, summaries of meetings, etc. d) Unpublished or duplicate literature.e) Unavailable full text.

Study selection
Two impartial reviewers (M.SH and N.N) reviewed the publications by reading the titles and abstracts after eliminating duplicate research.Next, the full texts of papers pertinent to the study's aims were carefully studied to establish eligibility.Disagreements about study selection were resolved by discussions between two researchers.In case of disagreement, it was referred to the third reviewer (S.S).

Data extraction and items
Further, the data from each finalized paper was extracted into a predefined Tables 2 and 3.The following data was extracted from each study: country, author, gender, age, type of study, outcome/side effect, inflammatory factor, dosage, type of administration, period, patient number in intervention groups and control, disease duration, and time of assessment/base treatment regimen (Tables 2 and 3).The data was retrieved independently by two reviewers who were well-versed in the matter.Any disagreements were resolved by discussion or referring to a third reviewer.

Assessment of risk of bias
Three reviewers assessed the risk of bias in the included studies with the standard summarized tool in the Cochrane Handbook [52].This tool assesses six domains related to the risk of bias (random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data,) and categorized studies by low risk, unclear risk or high risk of bias in each domain.For the random sequence generation domain, if the generation of a random sequence was non-randomly performed, the risk of selection bias was considered to be high.If a random component was described in the process of sequence generation, the selection bias would be considered as low risk; and if it was not explained in sufficient details, the selection bias was considered to be unclear.For the allocation concealment domain, if the participants or investigators could possibly foresee assignments, the risk of bias was considered to be high while if they could not foresee assignment, it was considered to be low.If the method of concealment was not described in sufficient detail, unclear risk of bias would be selected.For the blinding domain, it would be considered to be high if incomplete blinding or no blinding was done and the participants and personnel were aware of the interventions during the study, and low if blinding of investigators and participants was ensured.In case of insufficient information or not addressing this outcome, it was considered unclear.If there was no missing outcome data or in case of the existence of missing data, they were imputed using appropriate methods, this domain was considered as low; otherwise it was considered as high.Insufficient reporting of attrition/exclusions made this domain to be selected as unclear.In the Selective reporting domain, if not all of the pre-specified primary outcomes based on the study protocol was reported, high risk was selected.If the study protocol was available and all the outcomes was reported, it was considered as

Grading of the evidence
The JBI Grades of Recommendation framework provides a structured and transparent method for evaluating the quality of evidence and making recommendations based on that evidence.The JBI Grades of Recommendation framework consists of the following levels: Grade A: There is strong evidence to support the recommendation.This level is assigned when there is consistent highquality evidence from multiple studies.Grade B: There is moderate evidence to support the recommendation.This level is assigned when there is limited or inconsistent evidence from multiple studies or strong evidence from a single study.Grade C: There is weak evidence to support the recommendation.This level is assigned when there is limited evidence from a single study or expert opinion.Grade D: There is insufficient evidence to support or refute the recommendation.This level is assigned when there is a lack of available evidence [53] (Appendix).

Results
Based on the search strategy, 424 articles were obtained, and after removing duplicate articles, 103 studies remained.A complete review of the remaining articles led to the removal of 46 articles that did not have enough information or were irrelevant.Out of the remaining 57 articles, 12 articles were removed in the title and abstract section due to not mentioning the theoretical content or being unrelated.Finally, 44 articles was entered this systematic review and under the next subsection, which is categorized by the conditions, they will be further discussed.
The PRISMA flow chart is shown in Fig. 1.

Risk of bias within studies
The quality of the studies that are reported in our article were determined using the Cochrane Risk of Bias (Figs. 2 and 3).Accordingly, forty-two studies (95%) with low risk, one study (2.27%) with high risk, and one study (2.27%) with the unclear risk of bias for random sequence generation.For allocation concealment, fortytwo studies (95.45%) had unclear bias and two studies (4.5%) had low bias.
In the case of blinding of the participant: forty studies (90.90%) had unclear bias, and two study (4.54%) had low risk and 2 studies (4.54%) had high bias.Blinding of Fig. 1 PRISMA flowchart of the study outcome assessment was high risk in one study (2.27%), and low risk in three studies (6.81%) and unclear risk in forty (90.9%).In attrition bias, 63.63% of the studies were unclear risk, 34.09% were low risk, and 2.27% of the studies were high risk.
Meant for reporting bias, thirty-one studies (79.54%) had high risk, eleven studies (15.90%) had low risk and two studies (4.5%) had unclear risk.

Results of the individual studies RA
RA damages cartilage and bone by involving the joints, which has an incidence of about 0.5-1% and is known as one of the most currently known autoimmune diseases.The pain and swelling of the joints are caused by the initiation of pathologic alterations, via the entrance of B cells, macrophages and other factors to the synovial liquid [15,54].These alterations cause the overproduction of inflammatory factors including tumor necrosis factor (TNF), IL-1, PGE2, and cytokines bringing about chronic inflammation.GLA, the most abundant constituent of EPO, has anti-inflammatory effects through being converted to di-homo-γ-linoleic acid (DGLA) that stops the switching of AA to LT and the production of PGE-1, which plays important anti-inflammatory roles in RA [15,55].Five clinical trials with a study duration range of 3-12 months evaluated the effects of EPO on the improvement of RA.The patients received EPO in the form of oral capsules every day [15,[54][55][56][57].Three of them asserted that EPO was effective and showed significant improvements in symptoms of the disease, one of them stated that EPO had mild effects and others reported no significant effects [15,[55][56][57].In one of the studies, mild gastric discomfort (diarrhea, nausea, and pain) was reported as the side effect of EPO [54].Also, a letter claimed that a significant effect did not appear with consumption of EPO [58].In summary, it seems that EPO might be effective in terms of RA treatment.

Diabetes mellitus (DM)
DM has an increasing prevalence recently, and could affect different organs such as the kidney, brain, heart, and others, in chronic stages.It is diagnosed by different factors and is classified into 7 types.DM should be controlled by pharmacotherapy and lifestyle enhancement to reduce the risk of cardiovascular disease and the relative mortality [59].Some experiments claimed that abnormalities in red cell n-6 polyunsaturated EFAs and serum platelets could be increased in animals and humans with DM.The reduction of insulin levels blocks 6-desaturation and impairs the conversion of LA to GLA.This phenomenon has a key role in membrane structure and 6-desaturated metabolites, which are known as precursors of PGs and eicosanoids.All of these can lead to cardiovascular complications in the course of the DM disease.Consequently, it was suggested that high doses of LA and GLA could be effective in neuropathy improvement and cardiovascular disorder risk reduction [17].Two clinical trial studies reported the impact of EPO on DM.One of them prescribed EPO and placebo capsules to 22 patients for 6 months, twice daily where significant positive clinical and neurophysiological effects and polyneuropathy improvement were observed [17].Also, in the other study, with an intervention duration of 8 months, EPO capsules had beneficial effects on DM and vascular complications [17].In conclusion, a high dose of oral EPO was effective and improved the disease condition in DM.

Atopic eczema (AE)
AE is one of the most periodical inflammatory skin diseases showed a growing prevalence which could reduce the quality of life of the patients with this disease [60,61].Typically, AE is known as an immune system-related disease that mainly occurs in patients with a family or/and personal history of atopy and the lesions with different symptoms are its symptoms [62].Epidermal barrier deficiency is one of the most important reasons for the disease pathology and a route to its treatment.This can induce an increase in the permeability of the stratum corneum which causes more irritation by noxious substances and additive immunologic skin activation by increasing allergen entrance [49].The increasing level of LA and decreasing level of its derivatives confirmed that δ-6-desaturation is impaired [18].Indeed, inhibiting the cis-LA conversion to GLA induces the reduction of DGLA, the precursor of PGE1 that has an important role in the normal activity of T-lymphocytes (impaired T-lymphocyte function is clear in AE), and AA the precursor of PGE2 [18,63].AA and DGLA are appropriate and essential for the structure and normal activity of the skin [63] Even though topical steroids seem to be more effective than EFA supplements in AE, the final product of δ-6-desaturation could be of value in relief of AE [18,63].From 11 clinical trials that have been carried out in this case, 9 of them showed improvement and 3 of them claimed that EPO was not effective in the treatment process in AE (one of these studies used EPO as a topical emulsion despite other studies that used the oral capsules) [18,19,21,49,63,64].Consequently, it seems that EPO can be effective in AE through oral administration.

Chronic hands dermatitis
As far as we know, chronic hand dermatitis with a prevalence of 2-44%, has multifactorial pathologic reasons.Increasing the skin permeability irritates with materials that do not lead to any reaction in ordinary conditions.
The first line of treatment is recognition of the cause of the disease (possible allergens or irritating substances) and preventing their associations.Topical or systemic steroids and sometimes antibiotics are used as the drug treatments of the choice.Despite these pharmaceutical treatments, EPO supplementation could be effective via the role of GLA in increasing the water permeability in the epidermal barrier.In the only clinical trial that had been conducted on the chronic hand dermatitis cases having a duration of 24 weeks, 39 patients consumed oral EPO and the results did not show effectiveness [22].

Tardive dyskinesia (TD)
Tardive dyskinesia, one of the most serious complications in the patients with schizophrenia, has various prevalence related to age (from 4-5% in younger to about 24% in older people).Different factors such as age, smoking, psychiatric disorders, and others contribute to the risk of TD [17].EFAs have a non-negligible role in the structure and normal function of the membranes and also approximately 15-20% of the dry weight of the brain that is formed by them.EFAs derivatives (PGE1, PGE2) antagonize the dopamine function by cyclic adenosine -3' ,5'-monophosphate, thereupon, EPO could block the hyperactivity of dopamine causing TD by conversion to DGLA and AA (precursors of PGE1 and PGE2).In one study, 38 patients used EPO capsules for 32 weeks and marginal improvements were obvious, however, it was reported that the mentioned improvements were not clinically that important [47].

Psoriatic arthritis
Psoriatic arthritis, one of the common skin disorders, with a prevalence of 1-2%, impacts distal sites of hands and feet and has asymmetric joint distribution [48,65].The skin lesions, mainly appear in the hairline, natal cleft, ears, umbilical area, and groin.Also, nail lesions can lead to the true diagnosis [65].Impaired AA metabolism increases the AA and LTB4 levels in patients' lesions.Thus, EPO might improve the lesions.In a clinical trial with 38 participants and a duration of 12 months, patients consumed EPO capsules once daily, but all the factors measurements remained unchanged [48].In conclusion, it seems that despite the effective mechanism of EPO on psoriatic arthritis, it was not profitable for patients in this study.

Asthma
Asthma is categorized as one of the most common chronic inflammatory diseases with an increasing prevalence and morbidity that is activated by many different inflammatory cells in the airways.PGE1 as the final product of GLA conversion, is a bronchodilator and more stable than PGE2.Thus, EPO could create positive effects in this type of inflammatory disease [23].Two clinical trial studies with a duration of 8-16 weeks, have tested oral EPO capsules on asthmatic patients in both no significant positive results were obtained [23].We might conclude that EPO seems not to be effective in asthma.(As a result, despite the hypothesis that the drug is effective, no acceptable results were reported from these studies.

Polycystic ovary syndrome (PCOS)
Polycystic ovary syndrome, with an incidence of 5-15%, is generally caused through an impaired function of endocrine system (mostly ovarian hyper androgenism) in aged women, bringing about varied symptoms such as obesity, hirsutism, and acne [66].Assorted complications such as, high levels of low-density lipoproteins (LDL), low concentration of high-density lipoproteins (HDL), and high body mass index (BMI), accompany PCOS and increase the risk of type 2 diabetes mellitus (T2DM) and coronary heart disease (CVD) in patients suffering from PCOS.Supplements containing Vitamin D could recuperate lipoproteins concentrations and vitamin D levels in PCOS patients.More to the point, EPO has direct and indirect effects on synthesis of immune cells and eicosanoid.It has been reported that co-supplementation of vitamin D and EPO, in the form of oral capsules, on 60 patients and for 12 weeks, exhibited significant effects on the assessed parameters, claiming that EPO would be effective in PCOS [29].

Cystic fibrosis (CF)
Cystic fibrosis, a generalized endocrinopathy (affected exocrine glands), is recognized by the protein and fat malabsorption which can be caused via inhibition of pancreatic enzyme secretion leading the disease process into lung infection.Correspondingly, growth failure and steatorrhea are the diagnostic signs.Nutritional repletion, airway infection treatment, airway obstruction relief, suppression of inflammation, and lung transplantation (the main cause of morbidity and mortality in CF) are the choices of treatment [67].Given that EFA deficiency is widely reported in patients, predominantly due to the impaired function of pancreatic beta cells, EPO supplementation might be effective in CF.Nonetheless, in a clinical trial, with 16 participants and 12 months' duration of intervention, patients took oral EPO supplementations daily and no improvements were observed in the symptoms of CF.In view of that we might reject the theory of EPO effectiveness in CF [68].

Menopausal hot flashes
Peripheral vasodilation, increases blood skin flow coming with flushing, sweating (particularly in the chest, face, and neck), and chills.It has been anticipated that the duration of menopausal symptoms is about 4-20 years, and the prevalence is lower in Japanese and Chinese women.Despite the fact that every course of hot flashes usually takes a little time (1-5 min and sometimes 1 h), reduction in quality of life is the momentous issue.Therefore, different treatment options have been introduced such as behavioral treatment and drug therapies; clonidine, serotonergic agents, and etc., that are mainly divided into two categories, hormonal and non-hormonal medicines [69].EPO consumption is one of the nonhormonal ways of treatment in menopausal hot flashes.The mechanism of action for EPO is not clear yet in this case, but some references suggest it for alleviation of the symptoms.In a clinical trial, 56 aged women consumed t EPO capsules, every day for 6 weeks and significant improvement appeared in the symptoms deducing its effectiveness in menopausal hot flashes [30].

Hepatitis B
Hepatitis B is a chronic infection caused by the hepatitis B virus, with variable incidence, has effective and safe vaccine prevention.It is identified by hepatocellular damage, advanced fibrosis, and infiltration of inflammatory cells [70].As mentioned in the previous parts, EFAs have essential role in membrane functions and their supplementation would improve histological and biochemical parameters in liver disease.In a 12-month trial study, researchers evaluated oral EPO capsules effectiveness on 10 patients with hepatitis B and the achieved final result was not satisfactory [71].In conclusion, it appears that the offered treatment could not be acceptable for patients with hepatitis B.

Premenstrual syndrome (PMS)
Premenstrual syndrome are a set of symptoms including fatigue, breast tenderness, abdominal pain, depression, and etc., affect a large number of women before menses reducing the quality of life to a great extend [72].Despite the different treatment choices like hormones, anti-prostaglandins, vitamins, EPO supplementation is suggested owing to the PGE1 positive effect on abnormal sensitivity to the prolactin level in blood circulation in patients with PMS.A clinical trial study was designed for 38 patients and oral EPO capsules were tested for 6 months.The findings of that study revealed that there were no advantages in EPO consumption in comparison with the placebo [34].It looks like EPO might not be effective in reducing PMS symptoms.

Contact lens-associated dry eyes (CLADE)
A common complaint among contact lens users, which could be related to eye environment or the design and material of the lens.Variable symptoms such as blurred vision, eye fatigue, and prolonged dryness are experienced by patients more in the evening or night [73].Despite the beneficial treatments including anti-inflammatory and immunomodulatory drugs like cyclosporine, it has been reported that the final products of EPO metabolisms have anti-inflammatory effects on CLADE.In a controlled clinical trial study 52 participants with contact lens-associated dry eyes consumed oral capsules of EPO for 6 months.The outcome of the study showed no significant differences between the placebo and intervention groups [35].Accordingly it is suggested that EPO is not effective in this case.

Acne vulgaris
Overproduction of sebum affects the pilosebaceous unit and causes this chronic inflammatory disorder.Inflammatory (papules and pustules) and non-inflammatory lesions are the disease manifestations that are more aggregated in the neck, shoulders, face, upper chest, and back.Depending on acne severity, topical benzoyl peroxide, antibiotics and retinoids, and oral contraception or antibiotics, in combination or individually, could be effective [74].Isotretinoin consumption causes abnormalities in lipid profiles and LA reduction is noticeable.EPO consumption showed improvement in lipid profiles.
A clinical trial study was designed for 9 months, and biochemical parameters were assessed during EPO and placebo capsules consumption.In the end, improvement in lipid profile was clear in comparison with isotretinoin [42].After all, it is advocated that EPO supplementation could improve the disease condition in acne vulgaris.

Mastalgia
A common breast pain in women that mostly occurs before menstruation which is normal and physiological or is severe and nodularity which could reduce the quality of life and relationship with partner and the children and about 70% of women complain about it [28].Treatment choices includes danazol with the best response (70%) and low side effects (22%) but the most expensive price, bromocriptine with 45-47% of positive response, more side effects, and lower price in comparison to danazol, are available [42].Besides, decreasing the fat in dietary and stopping hormone replacement therapy and oral contraceptive consumption are the other effective supportive treatments [40].Despite all these therapy methods, EPO with uncommon adverse effects (2%), is the first choice for most patients because of the lower relapse rate and side effects of hormonal therapy [27]).On the other side, EPO affects the prostaglandin metabolism and can improve the mentioned complaint [25].Also, GLA deficiency is well-defined in mastalgia and since EPO is a rich source of GLA, it could be effective in the remedy for mastalgia [26].In 5 clinical trials, patients used oral EPO capsules for different durations and 4 of them claimed that significant improvement were obtained as a result [25,26,28,31].Also, a letter prescribed that oral EPO in a study with 135 patients had no significant effects on patients with mastalgia [75].Taken as a whole, we might claim that EPO supplementation might be effective in treatment of mastalgia.

Hypercholesterolemia
Impaired lipoprotein levels in plasma, increases the risk of cardiovascular and coronary heart disease (CHD).Some other risk factors such as diabetes, smoking, aging could play an important role in HDL reduction and LDL increment [75].Even though EPO metabolites can reduce cholesterol and LDL levels, n-6 metabolites (considerably DGLA and AA) deficiency is another risk factor for CHD and low levels of LA, explain it [75].Two controlled clinical trials studies were designed to assess the oral EPO consumption effect on lipoprotein level improvement and the results of them showed a reduction in LDL level [32,75].Consequently, EPO seems to be effective in down regulation of hypercholesterolemia.

Breast cyst
A common benign breast disorder, with an incidence of 70-90%, can be symptomatic or asymptomatic, small (microcyst) or large (macrocyst), single or multiple.Aspiration is one of the ways of treatment with periodic follow-up [32].A deficiency of LA metabolites caused by an abnormality in the fatty acid profile is clear in patients with macrocysts.So, EPO can be profitable in disease amelioration.In a study, daily usage of EPO and placebo capsules were evaluated on patients for 12 months and no differences were found between the 2 groups.In view of that, we might declare that EPO might not be effective in treatment of breast cyst [31].

Pre-eclampsia
Proteinuria and high blood pressure that occur in 3-5% pregnant women, is the main cause of mortality in patients.Variable risk factors including hypertension, previous pre-eclampsia, diabetes, autoimmune disorders, and chronic kidney disorders influence the disease affliction.Owing to the fact that the treatment cycle delivers different complications for the mother and fetus, prevention is vital during pregnancy [76].Reduction of prostacyclin (PGI2) and increment of thromboxane A2 (TXA2) is obvious in preeclamptic patients and AA could balance their levels [36].Also, low level of PGs (prostaglandins decrease vascular sensitivity) in patients is suggestive of EPO supplementation [37].In two studies, oral EPO capsules were evaluated on the preeclamptic patients and at the end of the intervention, no significant improvement was observed [36,37].It seems that the theory of EPO consumption as a way for pre-eclampsia treatment is deniable.

Multiple sclerosis (MS)
Evidently, MS is known as a common neurological disorder with an increasing prevalence that is multifactorial and many genes with different environmental factors such as smoking, obesity, and others, affect the affliction.Symptomatic pharmacotherapy to treat neurological dysfunction and MS-allocated remedies are available [77].Impairment in the Th1 (the interferon-gamma (IFN-γ) producer)/Th2 (interleukin IL-4 producer) balance is one of the etiological risk factors.On the other side, IL-17, the product of Th17, has a key role in MS pathogenesis, and the derived cytokines of Th1 and Th2, suppress the Th17 development [61].As it is stated in Traditional Iranian Medicine (TIM) warm temperament can create Th2-like immune responses, consequently, warm temperamented supplementation might be beneficial in autoimmune diseases that tend to Th1 immune responses (like MS).It has been reported that ω3-polyunsaturated fatty acids (ω3-PUFAs) can reduce IFN-γ generation in MS patients [45].Since the current treatments (like; IFN-β1a, and IFN-β1b) are expensive in comparison with their effectiveness and cause side effects, natural supplementation like EPO possibly would be advantageous [46].In a study, daily consumption of oral EPO and placebo was tested on 65 patients, and different factors were assessed after 6 months of drug administration.At the end of the study, all the assessed factors showed meaningful improvements [44][45][46]61].In a word, it seems that EPO can be helpful in MS.

Primary Sjogren's syndrome (SS)
Primary SS, is a common inflammatory disorder, with an incidence of about 0.04-4.8%,affecting the connective tissues with endocrine glands involvement [41,78].The regular symptoms are ocular and oral dryness and fatigue which is defined as mental or physical exhaustion and leads the disease process to the shrinked quality of life [41].Reduced PGE1 levels and major products of LA conversion in erythrocytes and the important roles of EFAs in cell membranes, justify the EPO efficiency in this disease [43,78].Three clinical trial studies assessed the effectiveness of daily usage of oral EPO capsules, nonetheless, the succeeded findings were not satisfactory, since just in one study EPO was effective [41,43,78].At long last, we might assume EPO could not be of value in primary SS.

Psoriasis
A chronic inflammatory skin disease, with environmental and genetic etiology factors, is divided into different types.Supposedly, stress, direct skin trauma, bacterial or viral infection, and other factors, are the affliction risk factors.Plaques, papules, and patchy lesions that sometimes are painful, manifest in different sizes on arms, legs, nails, and other parts of the body.Treatment should be individualized depending on the patient's condition and topical corticosteroids, tazarotene, vitamin D analogs, and others, are the medicinal treatments of the choices [79].Abnormality in EFAs levels and reduction in proinflammatory eicosanoids production and considering that the EPO derivatives prostaglandins have less inflammatory effects, EPO supplementation might be considered rewarding in psoriasis.In the study that was designed for 28 weeks, oral EPO capsules and a placebo were consumed by 37 psoriatic patients and no evident effect was observed [40].In conclusion, despite the effectiveness of EPO according to the mentioned hypothesis, in this study it was not effective in psoriasis.

Healthy volunteers
The effect of EFAs on epidermal atrophy, seed cake on the skin, and plasma fatty acid levels in humans, was evaluated on healthy volunteers.
• The effect of EFAs on epidermal atrophy: EPO and placebo in the form of topical formulation were tested on 24 healthy volunteers for 3 weeks and the result claimed that EPO was not effective on atrophy and no significant effect was observed [50].• The effect of seed cake on skin: a topical formulation of EPO and placebo were tested on 15 healthy volunteers for 6 weeks and significant positive effects were seen.EPO reduced skin irritation and improved the barrier function by hydrating the skin [69].• Plasma fatty acid levels in humans: this study was designed for 10 days and, oral EPO or placebo were taken by 76 healthy volunteers.The result of the study showed that EPO was harmful to inflammation due to the increasing levels of AA [80].

The strengths of the evidence
According to the JBI guidelines (Appendix), based on the available evidence, the recommendation for using EPO for inflammation control is weak.This conclusion is supported by the fact that the included studies are rather inconsistent and heterogeneous and there is lack of high quality studies.Larger sample sizes and narrower confidence intervals provide more precise estimates, increasing the certainty of evidence.

Discussion
As far as we know, O. biennis known as evening primrose is a medicinal plant from the Onagraceae family.The O. biennis seeds oil mainly contains active biological components including fatty acids, polyphenols, aliphatic alcohol, and sterols.It has been reported that the main components of EPO were LA (70-74%) and GLA (8-10%) demonstrate anti-inflammatory activity via different mechanisms.DGLA, the product of GLA, follows the two below mechanisms in exhibiting anti-inflammatory effects: 1. DGLA oxidation via lipoxygenase (15-LOX) and producing 5-hydroxyeicosatrienoic acid (15-HETrE) 2. DGLA conversion to PGE1 via cyclooxygenase (COX) Both of the final products in the above mentioned pathways possess anti-inflammatory activities.Likewise, 15-HETrE inhibit the AA conversion to LTA4 by blocking the 5-LOX enzyme and GLA reducing the following inflammation mediators' levels: IL-1β, IL-6, and TNF-α [81].
In this systematic review, based on our search strategy, from the initial 424 articles we selected 44 clinical trial studies in which they evaluated EPO effect on different types of inflammatory disorders.41 studies used EPO in the form of oral capsules and 3 studies tested the EPO in topical form.Although, about 28 clinical trial studies showed significant positive effects, nearly 10 types of diseases showed the opposite result.The most positive results were obtained in oral administration of EPO in AE, mastalgia and RA.As far as we know, depending on AE severity, variable regimens are recommended for treatment.Consumption of supplementations containing fatty acids (fish oil or EPO) besides, skin hydration, avoidance of skin stimulants and anti-inflammatory drugs including topical glucocorticosteroids and calcineurin inhibitors are the most advised treatments in AE.On the other hand, mastalgia is characterized with unclear etiology, decreasing the quality life of women specially before menstrual cycle.Unbalanced ratio of unsaturated (deficiency) and saturated fatty acids in patients, gives us a clue that PUFA supplementation could improve symptoms of disease, highlighting the role of EPO in treatment of mastalgia [82].Likewise, in the case of RA, combination of pharmacological and nonpharmacologic treatment base on the stage of the RA, reduce the symptoms and increase the quality of life.The supplementation of PUFAs could relief inflammation and based on the mechanism which has been mentioned in results, EPO seems to be much effective in RA.More to the point, it was publicized that EPO was also effective in a few reports including DM, hypercholesterolemia, MS, acne vulgaris, PCOS and menopausal hot flashes.To the best of our knowledge, omega-3 prevents the insulin resistance in DM through conversion to protectins and resolvins acting as an anti-inflammatory factor.Accordingly, unsaturated EFAs consumption such as EPO seems to be effective in DM [83].Furthermore, hypercholesterolemia cause inflammation via leading to cell wall diseases, large artery endothelial dysfunction.As regards, unsaturated fatty acids improve the dysfunction by unclear mechanism, so as, consumption of fatty acids supplementation like EPO might be effective in this kind of disease [84].
Acne vulgaris, a disease with various skin lesions and increasing prevalence, can cause decreased selfconfidence.Omega-6 supplementation like EPO cause effectiveness in patients by its antibacterial mechanism.Menopausal hot flashes can be reduced by PUFAs supplementation due to their effect on serotoninergic system, neuronal membrane and neurotransmitter function adjustment [85,86].Omega-6 fatty acids showed effectiveness in MS by experimental allergic encephalomyelitis suppression [87].However, attention should be paid to conducting more studies and evaluate the possible drug-interaction of the supplement with the prescribed drugs.
The oral form of EPO that was administered in those studies were formulated in the form of soft gels, containing 71-72% LA and 7-9% GLA in combination with antioxidants like vitamin E or tocopherol, mostly under the brand names of Efamol or Epogam.The EPO content in each soft gel was in the range of 250, 500, 1000 or 1300 mg.One study that was included in this review utilized EPO in the topical formulation as seedcake for skin hydration where EPO was extracted by ethanol (50%, v/v).Elsewhere, in 2 other studies EPO was administered to the AE patients and healthy volunteers (evaluating the skin atrophy) in topical pharmaceutical formulation in types of O/W and W/O emulsion [49].Also, oral liquid form of EPO in dark bottle, similar to Efamol manufacturer, was given to the patients with CF in one study [68].Accordingly, EPO in the all types of formulations in the studied diseases showed varied effectiveness due to the type of disease and dosage of the EPO that was used.
Concisely, EPO seems to be profitable in inflammatory diseases especially on RA, AE, mastalgia, hypercholesterolemia, DM, MC, TD, menopausal hot flashes, acne vulgaris, MS.One of the limitations of this study was the exclusion of some studies that might have useful content due to the reasons mentioned in the text.The next important object was that not all factors were included in the measurement of the inflammatory factors.Nevertheless, the advantage that the studies had was that the EPO in question was mainly formulated orally or topically and was used, which is very convenient for patients, considering this feature, it is suggested that researchers in the future on the formulation of the oral form sublingual form is also concentrated considering that have high bioavailability and may reach a satisfactory treatment result faster.It is also suggested to increase the number of participants and use different ages and genders in clinical trial studies.(This study contains the following limitations: • The mentioned studies in the text, have not assessed all the types of inflammatory factors.• The main form of EPO in the trials was capsule and the patients had to consume high dose of EPO to understand the effect.So, the high number of capsules could lead to decrease the patients' compliance.Consequently, loading more amounts of EPO in one capsule as a new formulation can be profitable in next studies.• Increasing the number, genders and range of the age of participants can improve the quality of next studies.

Conclusion
Overall, analyses of gathered data upon effectiveness of EPO supplementation in inflammatory diseases revealed that EPO publicized the most positive results in AE, mastalgia and RA, reducing disease symptoms and promoting improvements.Likewise, EPO was effective in DM, hypercholesterolemia, MS, acne vulgaris, PCOS and menopausal hot flashes in a few studies, However, we might not decisively claim its effectiveness in these diseases due to the low levels of evidences necessitating all-inclusive clinical trials.Nonetheless, it is recommended that future studies should be conducted to establish more comprehensive and generalizable results, further high-quality research is mandatory with larger sample sizes, longer interventions, with detailed investigation of influential inflammatory factors.Well-designed clinical trials combined with larger sample sizes and rigorous methodologies would provide a better understanding of the potential benefits and mechanisms of action for the EPO.These studies not only would be helping in determination of the appropriate dosage, treatment duration for any specific inflammatory diseases, but also could identify any potential side effects or interactions related to EPO with other medications.
and triglyceride concentrations did not change in either of the groups, but the serum HDL-cholesterol concentration increased slightly during olive oil treatment.EPO had no effect on the serum concentration of apolipoprotein A-I, whereas that of apolipoprotein B decreased in all three patients studied.Apolipoprotein A-I increased in all four studied patients treated with olive oil, whereas apolipoprotein

Fig. 2
Fig. 2 Risk of bias in randomized controlled trials using Cochrane risk of bias tool

Fig. 3
Fig. 3 Summary of risk of bias in selected studies (Cochrane Risk of Bias Tool for controlled trials)

Table 1
Different search terms and combinations used in designing the search strategy based on PICOs a a The OR Boolean operator was used between the terms in each column, while AND was used to combine the columns

Table 2
Characteristics of included clinical trial studies in the systematic review of EPO effectiveness (orally administered) on inflammatory diseases

Table 3
Characteristics of included clinical trial studies in the systematic review of EPO effectiveness (topically administered) on inflammatory diseases

Number of participants Female/Male ratio Age Type of intervention and dosage Duration of intervention Outcome side effect Intervention control Intervention control Intervention control
Moreover, if the study protocol was not available but it was clear that the published reports included all expected outcomes, it was also considered as low risk.In case of insufficient information to permit judgement it was unclear.Disagreements in risk of bias assessment were resolved through consensus.Quality assessment of included studies was conducted with the risk of bias table in RevMan 5.3 for RCTs.